Pulsed-neutron formation density

ABSTRACT

A method and related system of determining formation density by compensating actual inelastic gamma rays detected from a pulsed-neutron tool for the effects of neutron transport. The method and systems may model response of the tool and use the modeled response as an indication of an amount to compensate detected inelastic gamma rays.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention are directed to pulsed-neutron densitylogging tools. More particularly, embodiments of the invention aredirected to pulsed-neutron density logging tools that compensate forneutron transport effects.

2. Background of the Invention

Pulsed-neutron logging tools may be used in cased boreholes, and in somesituations pulsed-neutron logging tools may be operated withinproduction tubing. Pulsed-neutron logging tools operate on the principleof releasing high energy neutrons, on the order of 14 Mega electronVolts (MeV) into the formation. The high energy neutrons inelasticallycollide with other particles and thereby create gamma rays (known asinelastic gamma rays). Some of the inelastic gamma rays created by thecollisions make their way back to, and are detected by, gamma raydetectors on the logging tool. The ratio of received gamma rays betweena detector close to the pulsed-neutron source (the near detector) and adetector at some distance from the pulsed-neutron source (the fardetector) may be indicative of the bulk density of the formationsurrounding the borehole.

As neutrons lose energy through inelastic collisions (and the creationof inelastic gamma rays), they eventually reach an energy of thermalequilibrium, approximately 0.025 electron Volts (eV). When the neutronsapproach this thermal equilibrium energy, they may be captured bysurrounding atoms, and in the capture process a gamma ray may beproduced (known as a thermal capture gamma ray). Thermal capture gammarays too may propagate to the detectors on the tool.

When using a pulsed-neutron logging tool as a bulk density measurementdevice, inelastic gamma rays carry most of the information as to theformation bulk density. Thus, to determine a formation bulk densityusing a pulsed-neutron tool, it may be desirable to remove from thetotal received gamma rays the thermal capture gamma rays to be left onlywith inelastic gamma rays.

However, even after removing the thermal capture gamma rays from thetotal received gamma rays, bulk density measurements made with apulsed-neutron logging tool may not closely match actual bulk density.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawings inwhich:

FIG. 1 illustrates a logging system constructed in accordance withembodiments of the invention;

FIG. 2A illustrates inelastic gamma ray count rates of a modeledresponse for a near detector;

FIG. 2B illustrates inelastic gamma ray count rates of a modeledresponse for a far detector;

FIG. 2C illustrates inelastic gamma ray count rates of a modeledresponse after compensation for neutron transport effects;

FIG. 2D illustrates the relationship between density and modeledcompensated gamma gar count rates;

FIG. 3 illustrates a logging tool constructed in accordance withalternative embodiments of the invention;

FIG. 4A illustrates a modeled ratio of inelastic gamma ray count rates;

FIG. 4B illustrates a modeled ratio of thermal capture gamma ray countrates;

FIG. 4C illustrates a modeled ratio of inelastic gamma ray count ratesafter compensation for neutron transport effects; and

FIG. 4D illustrates a relationship between density and compensated ratioof inelastic gamma ray count rates in accordance with at least someembodiments of the invention.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. This document does not intendto distinguish between components that differ in name but not function.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . ”. Also, theterm “couple” or “couples” is intended to mean either an indirect ordirect electrical or mechanical connection, as the context may require.Thus, if a first device couples to a second device, that connection maybe through a direct connection, or through an indirect connection viaother devices and connections.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a logging system constructed in accordance with atleast some embodiments of the invention. In particular, a logging tool 8may comprise a tool body or sonde 10, placed within a well casing 12 ofa borehole. The well casing 12 may have cement 14 between its outerdiameter and the formation 16, and the borehole may be referred to as acased borehole. While some embodiments of the invention are directed tosystems for making bulk density measurements (hereinafter just“density”) in cased boreholes, the description in relation to a casedborehole should not be construed as limiting the embodiments of theinvention to only systems operating in a cased borehole environment. Thesonde 10 may be suspended within the borehole by an armoredmulti-conductor cable 18. The cable 18 may not only provide support forthe sonde 10, but also may couple a surface computer 20 to various gammaray detectors and neutron sources (discussed more fully below). Thesonde 10 may also be raised and lowered within the borehole by way ofthe armored multi-conductor cable 18.

A cased borehole may also comprise production tubing 22. The productiontubing 22 is of smaller diameter than the casing 12, and may be theroute by which hydrocarbons extracted from the formation are conveyed tothe surface. In accordance with at least some embodiments of theinvention, the logging tool 8 may be placed within the production tubing22, as illustrated in FIG. 1; however, placement of the logging tool 8within the production tubing 22 is not required as the logging tool 8may be equivalently placed within the casing 12 alone, or in an uncasedborehole (not specifically shown). Because production tubing may berelatively small in relation to the casing inside diameter, the loggingtool 8 may have a diameter slightly smaller than the internal diameterof the production tubing 22. For example, logging tool 8 may have adiameter of 1.6875 inches.

In accordance with at least some embodiments of the invention, thelogging tool 8 may comprise a neutron source 24 mounted to and/or withinthe sonde 10. The neutron source 24 is preferably capable of producinghigh energy neutrons, e.g. neutrons having energies of approximately 14MeV. The neutron source 24 may be capable of producing neutrons in apulsed fashion, possibly on command from the surface computer 20. Anyneutron source producing neutrons with sufficient energy and having arequisite size may be used. The logging tool 8 may also comprisedetectors 26, 28 mounted to and/or within the sonde 10. As illustrated,detector 26 may be relatively close to the neutron source 24, andtherefore may be referred to as the “near detector.” Likewise, detector28 may be spaced away from the source 24, and therefore may be referredto as the “far detector.” In accordance with at least some embodimentsof the invention, the near detector 26 may be approximately one footfrom the source 24, and the far detector 28 may be approximately two tothree feet from the source 24.

Neutron source 24 may generate neutrons having high energy, and theneutrons may interact with particles forming the production tubing 22,casing 12, cement 14, and/or formation 16 to produce gamma rays havingvarying energies. Thus, detectors 26 and 28 are preferably scintillationdetectors capable of detecting the presence of gamma rays, and alsodetecting energy of received gamma rays. Any suitable scintillationdetector may be used. Thus, sonde 10, neutron source 24 and detectors26, 28 may form the pulsed-neutron logging tool 8.

As alluded to in the Background section, high energy neutrons mayinteract and/or collide with other particles, and the collision processcreates gamma rays. FIG. 1 illustrates this process. In particular,arrows 30 and 32 illustrate high energy neutrons leaving the neutronsource 24. As the neutrons inelastically collide with other particles,gamma rays may be produced. In the case of a neutron illustrated by line30, gamma ray 34 produced by the illustrated inelastic collision mayproceed directly back to one of detectors 26, 28. In some cases, gammaray 34 may interact with other particles, and the interaction may createa different gamma ray that propagates to one of the detectors 26, 28. Asecond neutron, illustrated by line 32, may make an initial collision,and create a gamma ray 36. Gamma ray 36 exemplifies that not all gammarays created from the energy of the pulsed neutrons may proceed towardor be detected by the detectors 26, 28. As a neutron inelasticallycollides with other particles, its direction and energy may change, yetthe neutron may still have sufficient energy to create additional gammarays in secondary collisions. Line 38 illustrates a situation where aneutron has a first collision (creating gamma ray 36), and thereafterhas a second collision to create gamma ray 40, which in this example maypropagate towards one of the detectors.

As a neutron moves through the formation and interacts with variousparticles, it may lose energy until it reaches a low energy, on theorder of 0.025 eV. When a neutron reaches this low energy, also known asthermal energy, it may be absorbed into a surrounding atom, and theabsorption process may release a gamma ray. Dashed line 42 illustrates aneutron that has traveled into the formation and lost energy to thepoint that it reaches thermal energy, and is absorbed. The line 42 isdashed to exemplify that it is a resultant path, and the actual neutronpath may vary wildly by having many inelastic collisions, beforereaching the thermal energy state. The absorption of the neutron maycreate a gamma ray 44, which likewise may propagate towards and bedetected by the detectors 26, 28. Gamma rays created by the capture oflow energy neutrons may be referred to as thermal capture gamma rays.

Gamma rays created by the inelastic collisions are of primary interestin determining a density reading of a formation surrounding the loggingtool. That is, characteristics of inelastic gamma rays may carryinformation as to the density of the formation investigated. The thermalcapture gamma rays, by contrast, do not interact with the formation insuch a way that density may be accurately determined by measuring theircharacteristics. Thus, in accordance with some embodiments of theinvention, thermal capture gamma rays may be removed from the overalldetected gamma rays prior to a density determination.

Logging tools constructed in accordance with embodiments of theinvention may calculate a density based on the effects the density ofthe formation has on scattered gamma rays created during inelasticcollisions. However, the inventor of the present specification hasdetermined that accuracy of density readings based on the inelasticgamma rays is also affected by neutron transport characteristics of theformation. In other words, the interaction of the high energy neutronswith elements (such as in the casing or the formation) affect densityreadings calculated using back-scattered inelastic gamma rays. In orderto overcome the effect of neutron transport in determining density,embodiments of the invention may compensate detected inelastic gammarays for the neutron transport effects.

To elucidate the effects of neutron transport on density, the inventormodeled logging tool response as a function of porosity and density overa range of porosities and densities. In particular, using Monte Carloanalysis, or any suitable modeling system, logging tool response may bemodeled at various porosities, such as 5, 10, 20, 30 and 40 porosityunits (pu), and for density at ±0.2 grams per cubic centimeter (g/cc)around nominal density for each porosity. Modeling logging tool responseover a range of possible porosity and density values allows fordetermining a sensitivity of inelastic gamma ray count rates to densitysubstantially independent of neutron transport effects. In oneillustrative model, a logging tool may have a near detector spacing ofone foot and a far detector spacing of two feet. Further in theillustrative model, the number of high energy neutrons produced isknown, e.g. 10⁸ high energy neutrons. Thus, count rates for eachdetector 26 and 28 as a function of neutrons produced may be modeled andcompared. In alternative embodiments, ratios of detector count rates maybe used to make the model substantially insensitive to the number ofreleased neutrons.

For the modeled ranges of density and porosity discussed above, FIG. 2Aillustrates exemplary results of the analysis for the near detector. Inparticular, FIG. 2A illustrates near detector inelastic count rate (CR)on the ordinate, and density in g/cc along the abscissa axis. The countrate expressed in FIG. 2 may be a number of inelastic gamma raysdetected by the modeled logging tool for every 10⁸ neutrons produced inthe model. Each line of the family of lines in the plot area of FIG. 2Arepresents a different modeled porosity at nominal (roughly in themiddle of each line) and at ±0.2 g/cc from nominal for that porosity.FIG. 2A illustrates that for the particular spacing of approximately onefoot between the high energy neutron source and the near detector, theinelastic gamma ray count rate for the near detector shows a positivecorrelation to porosity, and relatively little correlation to density.

FIG. 2B illustrates a graph of the modeled response of the far detectorinelastic gamma ray count rate to density for the same model parameters.As illustrated in FIG. 2B, the far detector inelastic gamma ray countrate shows a sensitivity to both porosity and density.

In accordance with embodiments of the invention, a new measurementquantity is created from measured count rate values that is compensatedfor the neutron transport effects. More particularly, a compensatedinelastic count rate may be created that is compensated for neutrontransport effects. In at least some embodiments, the compensatedinelastic count rate may be determined using substantially the followingequation:CINEL=NINEL−X*FINEL  (1)where CINEL is a compensated inelastic count rate for a particularporosity and density, NINEL is a near inelastic count rate for theparticular porosity and density (values of FIG. 2A), FINEL is a farinelastic count rate for the particular porosity and density (values ofFIG. 2B) and X is a coefficient selected to substantially align thecompensated inelastic count rate values across all the modeled porosityand density values. Stated otherwise, X may be selected to substantiallyremove neutron transport effects evidenced as porosity sensitivity. Forthe exemplary values illustrated in FIGS. 2A and 2B, X may equal 3.4.Applying equation (1) above to each of the points in FIGS. 2A and 2B maythus create a plurality of compensated inelastic count rate values as afunction of density, as illustrated in FIG. 2C. Thus, for example, point50 in FIG. 2C may be calculated as point 52 in FIG. 2A minus the resultof 3.4 times the value of point 54 of FIG. 2B. Likewise, point 56 ofFIG. 2C may be calculated as the value of point 58 of FIG. 2A minus theresult of 3.4 times the value of point 60 in FIG. 2B. The value of thecoefficient multiplied with the far detector inelastic count rate ismerely exemplary, and may change depending on results of the model,which may be affected by parameters such as spacing between the neutronsource and each of the receivers.

FIG. 2D illustrates the information illustrated in FIG. 2C with theabscissa and ordinate axes swapped, and with a curve-fitted line to thecompensated inelastic count rate values. For the particular values ofthe model illustrated in FIGS. 2A–C, the line 62 that most closelyapproximates those values takes the form:DENSITY=3.27Ln(CINEL)+30.37  (2)Thus, a density determination in accordance with embodiments of theinvention follows a natural log of the compensated inelastic count rate.In a more general form, the relationship between the compensatedinelastic count rate and the density for embodiments using count ratesmay be expressed as:DENSITY=A*Ln(CINEL)+B  (3)where A and B are determined from curve fitting the compensatedinelastic count rates determined from the model.

Once the coefficients X (from equation 1) and A and B from equation 3are determined by modeling the tool response for a particular tool, thecoefficients and the equations may be utilized in compensating actualinelastic count rates from a logging tool and determining densitycompensated for the neutron transport effects. That is, the logging tool8 may be operated in an earth formation, and actual near and fardetector count rates may be determined. Using coefficient X determinedfrom modeling the logging tool response (the model run either before orafter use of the logging tool in earth formation), an actual compensatedinelastic count rate may be calculated. Using coefficients A and Bdetermined from modeling the logging tool response (again, the model runeither before or after use of the tool in earth formation), density maybe determined using the actual compensated inelastic count rate andequation 3 above.

FIGS. 2A and 2B illustrate modeled inelastic gamma ray count rates atvarious spacings from the neutron source. Any thermal capture gamma rayscreated in the model have been removed from the information illustratedin FIGS. 2A and 2B. Also, FIGS. 2A and 2B illustrate that densitysensitivity is better with increasing neutron source to gamma raydetector distance. With these points in mind, FIG. 3 illustrates alogging system 200 in accordance with alternative embodiments of theinvention. In particular, logging tool 80 may comprise a pulsed-neutronsource 24, operable to generate neutrons having energies ofapproximately 14 MeV. Logging tool 80 may also comprise a single gammadetector 82 at a spaced-apart location from the neutron source 24. Inaccordance with the alternative embodiments of the invention, thespacing between the neutron source 24 and the receiver 82 may be ofsufficient distance that the gamma rays received by the detector 82 showsensitivity to density in the surrounding formation. Thus, the spacingmay be a little as approximately 17 inches, and preferably is on theorder of approximately two to three feet.

In accordance with the alternative embodiments of the invention, thelogging tool 80 response may be modeled using Monte Carlo analysis, orany suitable analysis program or system, for various assumed porosities(and for each assumed porosity, various assumed densities). What may begenerated from the model may be a plot of inelastic gamma ray countrates as a function of density, similar to FIG. 2B. However, in theembodiments having only a single gamma ray detector, compensation forneutron transport effects may be based on parameters other than neardetector inelastic count rates.

The inventor of the present specification has found that thermal capturegamma rays detected are indicative of the neutron transport effect.Thus, in embodiments having only a single gamma ray detector,compensated inelastic gamma ray count rates may be calculated usingsubstantially the following equation:CINEL=INEL−Y*TC  (4)where INEL is the inelastic gamma ray count rate of the detector, TC isthe thermal capture gamma ray count rate of the detector, and Y is acoefficient selected to make the compensated inelastic count rate(CINEL) substantially insensitive to neutron transport effects. Statedotherwise, Y may be selected from modeled responses to substantiallyremove neutron transport effects evidenced in the inelastic count ratesas porosity sensitivity. Once the coefficient Y is determined from themodeled values, a characteristic equation relating the compensatedinelastic gamma ray count rate to the density similar to equation 3above may be determined. Thus, actual logging data, in particularinelastic gamma ray count rates, may be compensated by thermal capturegamma ray count rates (using the coefficient obtained from the model),and the formation density determined from the characteristic equationrelating the compensated inelastic count rate to the density.

Embodiments illustrated by FIGS. 1 and 3 may produce at least a countrate as a function of the number of neutrons released by the source 24.In particular, the inelastic count rates determined for the exemplarymodel are count rates for every 10⁸ neutrons released from the source24. Thus, in some embodiments of the invention, the source 24 may becapable of either: producing a regulated or known number of high energyneutrons; or a counter or detector may be used to count or approximatethe number of neutrons exiting the neutron source. In the case of acounter or detector, the detector may detect a number of neutronsexiting all or a part of a window where neutrons exit the neutron source24.

In alternative embodiments of the invention, the neutron source 24,while being capable of producing high energy neutrons, may be unable tocontrol the number of neutrons produced and/or may not be equipped tocount or approximate the number of neutrons produced. Thus, detectorcount rates (as a function of produced high energy neutrons) may not bedeterminable. In alternative embodiments, rather than inelastic gammaray count rates for near and far detectors, ratios of gamma raysdetected at the near and far detectors may be used, as ratios may besubstantially insensitive to actual neutron counts.

Thus, in accordance with alternative embodiments of the invention, aMonte Carlo model, or similar analysis, may be run to model ratios ofdetected gamma rays for a logging tool having particular source to nearand far detector spacings. A first ratio may be the ratio of inelasticgamma rays detected by the near and far detectors during the neutronburst. FIG. 4A illustrates a modeled response of the ratio of the nearand far inelastic count rates for a plurality of assumed porosities (andfor each assumed porosity, an assumed nominal density, ±0.2 g/cc). Thus,the family of lines (one each for each modeled porosity) shows asensitivity of the ratios of the inelastic gamma ray count rates todensity. The ratios also show some sensitivity to neutron transport, asevidenced by the separation between each line of the family of lines.

In embodiments where ratios of count rates are used, a second usefulratio may be the ratio of the count rates of thermal capture gamma raysdetected at the near and far detectors between neutron bursts. FIG. 4Billustrates a family of lines for a modeled ratio of thermal capturegamma ray count rates generated using the assumed porosity and densityvalues as discussed with respect to FIG. 4A. As illustrated in FIG. 4B,the ratio of thermal capture gamma ray count rate shows a correlation todensity, but the strongest correlation is with respect to porosity. Acompensated ratio may be determined, in this case using the ratio of theinelastic gamma ray count rate as illustrated in FIG. 4A and the ratioof the thermal capture gamma ray count rate as illustrated in FIG. 4B.In particular, the compensated ratio of the count rate in theseembodiments may be determined using the substantially the followingequation:CRINEL=RIN−Z*RNF  (5)where CRINEL is the compensated ratio of the inelastic gamma ray countrate, RIN is a ratio of the near and far inelastic gamma ray count rate,RNF is a ratio of the near and far thermal capture gamma ray count rate,and Z is a coefficient determined using the model which substantiallyaligns the compensated ratio of inelastic count rates. Stated otherwise,Z may be selected to substantially remove neutron transport effectsevidenced in the ratio of inelastic count rates.

FIG. 4C illustrates the compensated ratio of inelastic gamma ray countrates after application of equation 5 above. In particular, point 110may be calculated as point 112 (FIG. 4A) minus the result of 0.33 timespoint 114 (FIG. 4B). Likewise, point 116 may be calculated as point 18(FIG. 4A) minus the result of 0.33 times point 120 (FIG. 4B). Asillustrated in FIG. 4C, application of equation 5 to each of the datapoints generated from the model, with an appropriate coefficient, maysubstantially align the various points. For the values modeled in FIGS.4A and 4B, Z may be 0.33; however, having Z equal 0.33 is merelyexemplary.

FIG. 4D illustrates the compensated data points of FIG. 4C with theabscissa and ordinate axes swapped, and with a curved-fitted linethrough the compensated values. Much like the compensated inelasticcount rate of FIG. 2D, the relationship between the density and thecompensated ratio of the inelastic count rate of FIG. 4D takessubstantially the form:DENSITY=M*Ln(CRINEL)+N  (6)For the particular simulation used to generate the FIGS. 4A–D,coefficient M may take a value of 4.25 and the coefficient N may take avalue of −4.59.

Thus, a Monte Carlo, or similar analysis, may be run to determine thetool response for various assumed porosities. Once the ratio of the nearand far inelastic gamma ray count rates, and the ratio of the near andfar thermal capture gamma ray count rates are determined for the modeledparameters, the coefficient needed to create the compensated ratio ofinelastic count rate may be determined (along with the coefficients ofthe characteristic equation). The logging tool may then be used in anactual formation to obtain a ratio of actual near and far inelasticcount rates, and also to determine a ratio of actual near and farthermal capture gamma rays. Using the actual ratios and the coefficientZ determined in the modeling process, a compensated ratio may becreated, which may be applied to equation 6 (along with the coefficientsof equation 6 determined from the model) to calculate a density.Determining density in this manner, the neutron transport effects may besubstantially reduced, thus increasing the accuracy of the densitydetermination using a pulse-neutron logging tool.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. For example, modeling toolresponse need not be performed prior to use of the logging tool in anactual formation. In fact, the methods of compensating for neutrontransport effects discussed herein may be used on existing data.Further, determining the coefficients of the equations may beaccomplished from laboratory measurements (physical models), or evenfrom field log measurements where a comparison density log is available.Although FIG. 1 illustrates a surface computer, any processor,regardless of its location, e.g. on the surface, within the sonde, or ata remote location, may be used. It is intended that the following claimsbe interpreted to embrace all such variations and modifications.

1. A method comprising compensating an actual inelastic gamma ray countrate detected by a pulsed-neutron logging tool for the effect of neutrontransport by modifying the actual inelastic gamma ray count rate in anamount proportional to the effect of the neutron transport to create acompensated inelastic gamma ray count rate.
 2. The method as defined inclaim 1 further comprising calculating bulk density of a formation usingthe compensated inelastic gamma ray count rate.
 3. The method as definedin claim 1 wherein compensating actual inelastic gamma ray count rate tocreate the compensated inelastic gamma my count rate further comprisescreating the compensated inelastic gamma ray count rate using an actualinelastic gamma ray count rate at a near detector, an actual inelasticgamma ray count rate at a far detector, and a correction coefficient. 4.The method as defined in claim 3 further comprising: modeling inelasticgamma ray count rates for a near detector of the pulsed-neutron loggingtool; modeling inelastic gamma ray count rates for a far detector of thepulsed-neutron logging tool; and determining the correction coefficientfrom the modeling used to combine the near and far inelastic gamma raycount rates to create compensated modeled inelastic gamma ray countrates that are substantially free from neutron transport effects.
 5. Themethod as defined in claim 3 wherein creating the compensated inelasticgamma ray count rate using an actual inelastic gamma ray count rate at anear detector, an actual inelastic gamma ray count rate at a fardetector, and a correction coefficient further comprises usingsubstantially the following equation:CINEL=NINEL−X*FINEL where CINEL is the compensated inelastic count rate,NINEL is the near inelastic count rate, FINEL is the far inelastic countrate, and X is the correction coefficient.
 6. The method as defined inclaim 1 wherein compensating the actual inelastic gamma ray count rateto create the compensated inelastic gamma ray count rate furthercomprises creating the compensated inelastic gamma ray count rate usingan actual inelastic gamma ray count rate by a detector of thepulsed-neutron logging tool, an actual thermal capture gamma ray countrate by the detector, and a correction coefficient.
 7. The method asdefined in claim 6 further comprises: modeling inelastic gamma ray countrates for the detector of the pulsed-neutron tool; modeling thermalcapture gamma ray count rates for the detector; and determining thecorrection coefficient used to combine the inelastic gamma ray countrates and the thermal capture gamma ray count rates from the modelingthat creates compensated modeled inelastic gamma ray count rates thatare substantially free from neutron transport effects.
 8. The method asdefined in claim 6 wherein creating the compensated inelastic gamma raycount rate using the actual inelastic gamma ray count rate by thedetector of the pulsed-neutron logging tool, the actual thermal capturegamma ray count rate by the detector, and the correction coefficientfurther comprises using substantially the following equation:CINEL=INEL−Y*TC where CINEL is the compensated inelastic count rate,INEL is the inelastic gamma ray count rate, TC is the thermal capturegamma ray count rate, and Y is the correction coefficient.
 9. The methodas defined in claim 1 wherein compensating the actual inelastic gammaray count rate to create the compensated inelastic gamma my count ratefurther comprises creating a compensated ratio count rate using ratiosof an actual inelastic gamma ray count rate at near and far detectors ofthe pulsed-neutron logging tool, ratios of an actual thermal capturegamma ray count rate at the near and far detectors, and a correctioncoefficient.
 10. The method as defined in claim 9 further comprising:modeling ratios of inelastic gamma ray count rates for a near and fardetector of the pulsed-neutron logging tool; modeling ratios of thermalcapture gamma ray count rates for the near and far detectors; anddetermining the correction coefficient used to combine the ratio of theinelastic gamma ray count rates and the ratio of the thermal capturegamma ray count rates from the model to create compensated modeled ratiocount rates that are substantially free of neutron transport effects.11. The method as defined in claim 9 wherein creating a compensatedratio count rate using ratios of an actual inelastic gamma ray countrate at near and far detectors of the pulsed-neutron logging tool,ratios of an actual thermal capture gamma my count rate at the near andfar detectors, and a correction coefficient further comprises usingsubstantially the following equation:CRINEL=RIN−Z*RNF where CRTNEL is the compensated ratio of the inelasticcount rate, RIN is the ratio of the near and far inelastic count rate,RNF is the ratio of the near and far thermal capture gamma my countrate, and Z is the correction coefficient.
 12. A logging systemcomprising: a logging tool comprising: a sonde operable within a borehole; a neutron source coupled to the sonde, the neutron source operableto produce high energy neutrons; a near gamma ray detector coupled tothe sonde at a first distance from the neutron source; and a far gammaray detector coupled to the sonde at a second distance from the neutronsource, the second distance greater than the first distance; a processorcoupled to the neutron source and the near and far gamma my detectors;and wherein the processor calculates a compensated inelastic gamma raycount rate for a formation surrounding the logging tool, the compensatedinelastic gamma ray count rate calculated by modifying actual inelasticgamma ray count rates in an amount proportional to the effect of neutrontransport.
 13. The logging system as defined in claim 12 wherein neutronsource of the logging tool further comprises: a neutron detectoroperable to determine a number of at least a portion of the produced;and wherein the neutron source at least partially controls the number ofneutrons produced during each activation of the neutron source.
 14. Thelogging system as defined in claim 13 wherein the processor calculates abuilt density of the formation using the compensated inelastic countrate calculated as a function of count rates of the near and far gammamy detectors per unit number of neutrons exiting the neutron source, anda correction coefficient.
 15. The logging system as defined in claim 14wherein the processor calculates the built density by applying thecompensated inelastic count rate to a relationship between bulk densityand compensated inelastic count rate determined from a modeled loggingtool response.
 16. The logging system as defined in claim 12 wherein theprocessor calculates a bulk density of the formation using thecompensated inelastic gamma ray count rate being a compensated ratio ofthe inelastic count rate calculated as a function of the ratio of countrates of the near and far gamma ray detectors and a correctioncoefficient.
 17. The logging system as defined in claim 16 wherein theprocessor calculates the bulk density of the formation by applying thecompensated ratio of the inelastic count rate to a relationship betweenbulk density and compensated inelastic count rate determined from amodeled tool response.
 18. A logging system comprising: a logging toolcomprising: a sonde operable within a bore hole; a neutron sourcecoupled to the sonde, the neutron source operable to produce asubstantially known quantity of high energy neutrons; a gamma mydetector, the gamma ray detector disposed within the sonde at a spacedapart location from the neutron source; and a processor coupled to thelogging tool; wherein the processor is operable to compensate aninelastic gamma ray count rate for neutron transport effects to create acompensated inelastic gamma my count rate.
 19. The logging system asdefined in claim 18 wherein the processor compensates the inelasticgamma ray count rate for neutron transport effects by computing thecompensated inelastic gamma ray count rate using an inelastic gamma mycount rate of the gamma my detector, a thermal capture gamma ray countrate of the gamma my detector, and a coefficient.
 20. The method asdefined in claim 11 wherein creating further comprises usingsubstantially the following equation:CRINEL=RIN−Z*RNF ^(X) where CRINEL is the compensated ratio of theinelastic count rate, RIN is the ratio of the near and far inelasticcount rate, RNF is the ratio of the near and far thermal capture gammamy count rate, Z is the correction coefficient and X=1.